Process for chromium electroplating using electrolytic solutions containing trivalent chromium

ABSTRACT

A PROCESS FOR MINIMIZING THE PRESENCE OF HEXAVALENT CHROMIUM IONS IN A CHROMIUM ELECTROPLATING SOLUTION WHICH COMPRISES ADDING A BARIUM SALT TO THE SOLUTION.

United States Patent Office 3,649,479 Patented Mar. 14, 1972 Int. Cl. (5231; /06

US. Cl. 204-51 1 Claim ABSTRACT OF THE DISCLOSURE A process for minimizing the presence of hexavalent chromium ions in a chromium electroplating solution which comprises adding a barium salt to the solution.

This invention relates to a process for chromium electroplating using electrolytic solutions containing trivalent chromium.

It is well known that the electrolytes containing trivalent chromium used for electroplating purposes are subject to variations in the yield of the chromium deposited by the electric current. These variations are due to the formation of parasitic hexavalent chromium in the electrolytic bath, the formation of which increases during the course of the operation. The hexavalent chromium diminishes the yield and reduces the quality and the purity of the chromium plating, thereby making the electrolyte unfit for further plating chromium. The work of the ap plicant has confirmed these findings. In effect, when hexavalent chromium is introduced into electrolytic solutions containing trivalent chromium, the yield of the chromium plating is lowered and eventually becomes zero. This is the case even when the quantity of hexavalent chromium introduced into the solution is extremely weak as the applicant noticed that ions Cr VI are active as soon as they are detectable by the chlorimetric diphenylcarbazide method. This problem exists for all types of chromium electroplating of this type by tank, brush platmg, etc.

The applicant has discovered that by introducing into the electrolytic (plating) bath a compound for rendering insoluble the hexavalent chromium (said hexavalent chromium usually being present in the form of a chromium anhydride C-rO the amount of the hexavalent chromium dissolved in the bath can be reduced or substantially reduced, whereby the electroplating can then be conducted normally with the electrolytic solutions of chromium (III) providing stable electrochemical characteristics.

Consequently, the present invention has for its objective a process for substantially eliminating or limiting the presence of soluble hexavalent chromium in electroplating solutions containing trivalent chromium by adding, before or during the electroplating operation, a suitable quantity of a barium compound. The barium compound can be added in soluble or as a solid compound. Said addition can be continuous. The amount of the barium compound added will be selected so as to enable the plating operation to continue in an effective manner. The insoluble chromate of barium thereby formed are eventually removed from the electroplating bath by continuous or noncontinuous filtration or by any other means, such as centrifuging, decanting, etc.

According to the present invention, the barium salt will be utilized preferably in the form of a convenient salt which is soluble in the electrolytic solution, such as: chloride, nitrate, formate, acetate, oxalate. The quantity of the barium compound added is selected so that the amount of barium ions in the electroplating bath ranges from 10% to the stiochiometric amount in relation to the chromium ions in the electroplating bath (Ba/Cr=1/l0 to 1/1) For example, it has been noted that in the case of an electrolysis using an insoluble anode, the inhibition due to barium ions is increased, a quantity of barium ions equal to about 10% of the stoichiometric amount to the chromium ions in solution being sufiicient to prevent the presence of chromium (VI) in the electroplating solution. The barium salt can be added in an amount to give an excess of barium ions in the solution but this provides no advantageous result and is not necessary.

However, the process according to the present invention is in principle limited to the use of electrolytic solutions of trivalent chromium, free of sulfate ions and which have a pH above 2.5. If sulfate ions were present, there would be formed a BaSO, precipitate, which is more insoluble than BaCrO, and eventually if these sulfate ions are present such as impurities e.g. it is necessary to take it into account. Barium chromate is soluble in moderately strong acid (pH 2.5) so the electroplating solution must not be strongly acid.

The present invention also has for its ojective the products obtained by such an electrolytic treatment.

The following examples, which have no limiting significance, further illustrate the present invention.

EXAMPLE I The bath consisted of the following salts dissolved in distilled waterto make a final volume of 1,000 ml:

266 g. of chromium chloride hydrateCrCl -6H O 160 g. of potassium chlorideKCl 244 g. of barium chloride hydrateBaCl -2'H O The electroplating was carried out under a potential of 8-12 v., with a current density of from 2-50 amp./ dm. (decimeters using a chrome anode and an operating temperature below 60 C. The bath was continuously filtered. The plated products were of good quality.

EXAMPLE II A procedure for brush tampon plating etc. type was carried out using a platinum anode and a density of current of 150 amp./dm. Chromium metal was effectively plated using the following plating solution.

In 900 ml. of water at 60 C. the following constituents were dissolved in the following named order:

' g. of ammonium acetateNH C H O 60 g. of barium acetateBa('C H O -H O to make a total volume of one liter.

The quantity of barium present was suflicient to hinder the formation of chromium (VI). It corresponded to about 20% of the stiochiometric quantity equivalent to the chromium in the solution. The presence of the ammonium salts hindered the precipitation of barium oxalate.

Although particular preferred embodiments of the invention have been disclosed above for illustrative purposes, it will be understood that variations or modifications thereof which lie within the scope of the appended claim are fully contemplated.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a chromium electroplating process utilizing an electroplating solution of chromium (I-II) salt, the solution being substantially free of sulfate ions and having a pH above 2.5, the improvement comprising the step of maintaining in said electroplating solution a concentration of barium ions to chromium ions in the range of 1:10 to 1:1 in order to precipitate hexavalent chromium ions formed in the solution during the plating process.

References Cited UNITED STATES PATENTS 1,838,777 12/1931 McCullough et a1. 204-51 2,587,651 3/1952 Rojas 20451 2,841,541 7/1958 Smith 20451 4 11/ 1963 Safranek, In, et a1. 20443 12/1968 Nelson, Jr. 20451 7/1969 Dettner 204-51 5/1970 Nelson, Jr. 204-51 11/ 1934 Atkinson 204263 X FOREIGN PATENTS 4/1926 Great Britain 235 6 OTHER REFERENCES A. G. Gray, Modern Electroplating, pp. 148-150 GERALD L. KAPLAN, Primary Examiner 

